The present invention relates to an oxygen sensor, and particularly to an oxygen sensor suited for measuring the concentration of an oxygen gas contained in an exhaust gas of an automobile.
An oxygen sensor of this kind has a heater attached to a cell, and as for its manufacturing technique, the following methods are proposed, one of which is described in Japanese Utility Model Laid-Open No. 130,261/1983 in which green sheets of ZrO.sub.2 and Al.sub.2 O.sub.3 as cell elements and an electric insulating substrate, respectively are formed, electrodes, heaters, etc. are printed on the green sheets, and they are laminated and then sintered simultaneously without using a bonding agent. The other method is disclosed in Japanese Patent Laid-Open No. 154,451/1980 in which a cell formed of a thin or thick film of stabilized ZrO.sub.2 is superposed in a layer on a heater of an Al.sub.2 O.sub.3 base sintered beforehand and then is sintered to be joined to the latter by heat treatment also without using a bonding agent.
Since the ZrO.sub.2 and Al.sub.2 O.sub.3 layers have different thermal expansion coefficients, stresses are caused in the joint interface between the two layers by a difference in thermal expansion due to a heat cycle between the operating condition (600.degree. to 900.degree. C.) of an apparatus and the stationary condition (a room temperature) thereof, and this tends to cause exfoliation, cracking or the like. Moreover, simultaneous sintering of both materials, which is often the practice, causes problems such as insufficient strength of Al.sub.2 O.sub.3 due to insufficient sintering thereof, and a reduction in electric resistance due to adding too much sintering aid.